I. Field of the Invention
The present invention relates to satellite communications systems. More specifically, it relates to a compact antenna that communicates with earth orbiting satellites. Still more specifically, the invention relates to a compact antenna having a plurality of antenna elements to provide maximum gain through all angles between about 10 and 90 degrees of elevation.
II. Description of the Related Art
In communication systems, low earth orbit (LEO) and medium earth orbit (MEO) satellites are increasingly being viewed as having great potential in satellite telephone communications. For example, plans are underway to put up constellations of LEO satellites that operate close to the earth's surface, in the range of 400 to 1000 miles above the earth's surface. Most commercial satellites today that handle long-distance phone calls, relay cable television, and provide direct-broadcast television are in geostationary earth orbit (GEO), 22,300 miles above the earth's surface. At that altitude and location, they move around the earth at the same rate as the earth rotates about its axis, making them appear to hover over the same point and allowing them to act like very tall transmission towers. The problem, however, is that relatively few positions are available on the geostationary arc, which is precisely above the equator. It takes powerful, costly rockets to boost heavyweight satellites into that position. And because they are so far away, it takes a quarter of a second for a signal to travel from earth to a geostationary satellite and back to earth. This delay is not material for television broadcasts or data transmissions. However, for voice phone calls, this delay can interfere with a lively back-and forth conversation.
There is practically no limit, however, to the number of satellites that can be placed in low earth orbit. Since LEO satellites are close to earth, there is essentially no delay to the signal, less radio energy is required, and phone batteries can be smaller. The satellites themselves can be smaller, cheaper, and easier to launch. Several satellite services operate using LEO satellites. Other satellite services expect to place satellites in orbit 6430 miles above the earth's surface in a Medium Earth Orbit (MEO). At that altitude, only 10 satellites are required, and the signal delay is minimal. Some of the techniques for managing big constellations, dozens or hundreds of satellites working together, were hypothesized initially for the so-called "Star Wars" missile-defense schemes.
A problem facing users of fixed phones that will communicate directly with LEO and MEO satellites is their ability to receive weak signals transmitted by the orbiting satellites. Currently, high gain dish antennas are used to communicate with LEO and MEO satellites. These satellites must be tracked in real-time in order to provide continuous, uninterrupted communications. This requires that the dish be mounted on a gimbal so that it can be electro-mechanically steered. Complicated dual-mode microwave feed systems and specialized auto-tracking receivers are needed to track the satellites. Complex algorithms have been developed to enable the dish to track the satellite. Also, complex electronics and control circuitry are required for this purpose. All these make the dish antenna very expensive. Use of these antennas for mobile and fixed phones would be cost prohibitive and impractical due to their complexity and the difficulty in calibrating and maintaining the systems.
One alternative to the steered dish antenna is an omni-directional antenna. The omni-directional antenna, however, typically has a lower gain than a steered dish antenna. In addition, the omni-directional antenna suffers from an environmental effect, known as "specular reflection," which causes signal fading. At angles near the horizon, the antenna picks up signals, in phase and out of phase, reflected from the ground. Thus, when a satellite is at or near the horizon, the antenna picks up undesired signals being reflected from the ground, which may, if out of phase, add destructively to signals received directly from a satellite.
Another alternative to the dish antenna is a beam forming array of smaller antenna elements, where a beam from each antenna element is electronically steered to track a moving satellite. This type of system is commonly known as a rotatable phased array system. Each antenna element, however, requires its own electronic and control circuitry, which makes a phased array system complex and expensive. Additionally, complex algorithms are required to enable the beams to track the moving satellite.
As a result, many in the satellite communications environment have realized that there is a need for a compact, low cost antenna which, at high gain, tracks LEO and MEO satellites. There is also a need for an antenna that provides improved immunity from environmental effects, such as specular reflections. There is also a need for an antenna that requires a minimum number of elements and thus less electronic and control circuitry. Furthermore, there is a need for an antenna for satellite communications systems that offers good signal reception without complex RF processing hardware or sophisticated beam alignment systems.
The present invention addresses the above-described needs. Briefly stated, the present invention is directed to a compact, low cost antenna that communicates with low and medium earth orbit satellites and provides maximum gain through all angles between 10 and 90 degrees of elevation. The present invention provides a simple, inexpensive alternative to expensive, complicated antennas currently being used to communicate with these satellites.